1. Technical Field
The present invention relates to the linearization of radiofrequency (RF) power amplifiers. It finds applications, in particular, in the RF transmitters of the mobile terminals of digital radiocommunication systems. It may also be applied in the RF transmitters of base stations in particular during the first start-up of such a station.
2. Related Art
In current digital radiocommunication systems, one seeks to send information with a maximum throughput in a given RF frequency band which is assigned to a transmission channel (hereinbelow radio channel). To do this, the modulations that have been used for a few years comprise a phase or frequency modulation component and an amplitude modulation component.
Moreover, radio channels coexist in a determined frequency band allotted to the system. Each radio channel is subdivided into logical channels by time division. In each time interval (or time slot), a group of symbols called a burst or packet is transmitted.
It is necessary to take care that, at each instant, the power level transmitted in each radio channel does not jam the communications in an adjacent radio channel. Thus, specifications prescribe that the power level of an RF signal transmitted in a determined radio channel be, in an adjacent radio channel, less for example by 60 dB (decibels), than the power level of the RF signal transmitted in said determined radio channel.
It therefore turns out to be necessary that the spectrum of the signal to be transmitted, which results in particular from the type of the modulation employed and the binary throughput, not be deformed by the RF transmitter. In particular, it is necessary that the RF transmitter exhibit a characteristic of output power as a function of input power, which is linear.
However, the radiofrequency power amplifier (hereinafter RF amplifier) present in the RF transmitter has a characteristic that is linear at low output power but nonlinear as soon as the power exceeds a certain threshold. It is also known that the efficiency of the RF amplifier is all the better when working in a zone close to saturation, that is to say in the nonlinear regime. Thus, the need for linearity and the need for high efficiency (to save on battery charge) compel the use of linearization techniques to correct the nonlinearities of the RF amplifier. Two of the techniques most commonly employed are baseband adaptive predistortion and the baseband Cartesian loop.
In the Cartesian loop technique, the signal to be transmitted is generated in baseband in the I and Q format. Additionally, a coupler followed by a demodulator makes it possible to tap off a part of the RF signal transmitted and to transpose it to baseband (downconversion), in the I and Q format. This baseband signal is compared with the baseband signal to be transmitted. An error signal resulting from this comparison drives a modulator, which provides for the transposition to the radiofrequency domain (upconversion). The output signal from the modulator is amplified by an RF amplifier which delivers the RF signal transmitted.
In the baseband adaptive predistortion technique, the signal to be transmitted is generated in baseband, in the I and Q format, and predistorted via a predistortion device. Then, this signal is transposed to the RF domain by virtue of an RF modulator. Next, it is amplified in an RF amplifier. A coupler followed by an RF demodulator make it possible to tap off a part of the RF signal transmitted and to transpose it to baseband, in the I, Q format. This baseband demodulated signal is digitized and compared with the baseband signal to be transmitted. An adaptation of the predistortion coefficients, carried out during a phase of training of the predistortion device, allows the demodulated I and Q format signal to be made to converge to the I and Q format signal to be transmitted.
In both techniques, a part of the signal transmitted is tapped off at the output of the RF amplifier so as to compare it with the signal to be transmitted. As a result, linearity is not obtained immediately but only after a certain time, required for the convergence of the linearization device. The signal transmitted has, during a period corresponding to the phase of training of the linearization device, a spectrum that is widened by the uncorrected nonlinearities. It may not comply with the constraints on the spectrum of the signals transmitted. This remark applies admittedly more to adaptive predistortion than to the Cartesian loop, even if the latter requires, in order to ensure its stability, initial adjustments of phase and of amplitude levels akin to training.
Several procedures have been proposed in the prior art for remedying this problem.
The procedure disclosed in WO 94/10765 relies on the transmission by the transmitters of the system of particular sequences, so-called linearization training sequences, during linearization training phases. More particularly, training sequences are transmitted in an isolated manner in time intervals forming a particular logical channel of the radio channels, which is dedicated solely to linearization. As all the transmitters of the system transmit their respective training sequence at the same time, the communications are not disturbed by interference between the radio channels which may possibly occur at this moment. It is therefore not necessary to prevent interference between the radio channels of the system.
However, this procedure has several drawbacks. Firstly, it requires prior synchronisation of all the transmitters so that the latter transmit their respective linearization training sequence in the logical channel dedicated to linearization. Moreover, no sending of data can occur in the time intervals of this logical channel. Furthermore, at the start of each transmission or in the event of a change of radio channel, the transmitter is compelled to wait for the next time interval of the logical channel dedicated to linearization, unless the system is made considerably more complex. This is why the temporal spacing between two time intervals of said logical channel cannot exceed a second, so as to guarantee a certain quality of service (QoS). This technique is therefore very prejudicial to the spectral efficiency of the radiocommunication system. Finally, since no particular precaution is taken to avoid out-of-band transmission during the linearization training phases, this technique may generate interference in respect of the transmitters of the other radiocommunication systems, which do not conform therewith.
Another procedure, disclosed in U.S. Pat. No. 5,748,678, provides for the use during the linearization training phases of a second modulator having half the throughput of the modulator normally used for the transmitting of useful data. This second modulator generates a signal which possesses the same depth of amplitude modulation but a spectral width that is divided by two with respect to the signals transmitted outside of the linearization training phases. This makes it possible to avoid jamming the adjacent radio channels by the signals uncorrected for the nonlinearities which are transmitted during the linearization training phases.
However, this procedure is fairly complex to implement since it requires a second modulator, as well as associated filters or the use of adaptive filters. This second modulator serves only during the linearization training phases, that is to say for a very small fraction of the time. Specifically, when the amplifier has been linearized initially, it suffices to correct any drifting of its characteristics. The linearization devices of the type alluded to in the introduction may deal therewith in the course of the transmitting of useful data (at the normal throughput). The overhead related to this corresponding second modulator is therefore hardly justified.